The present invention relates to a thin film capacitor using silicon dioxide as a dielectric, and more particularly to a thin film capacitor which includes a thin film layer of silicon dioxide having excellent electric characteristics and which is suitable for use in high integrated circuits.
Capacitors are passive elements or parts indispensable to electric and electronic circuits and perform various functions in the circuits in combination with resistor, coil, IC and the like. A number of main functions in electronic circuits, including filter for removing noises in power circuit, oscillation circuit for stably generating necessary signals, tuning circuit for selecting necessary signals and by-pass circuit for temperature compensation of circuits, largely depend on the characteristics of capacitors.
In general, capacitors have a structure that a dielectric is sandwiched between a pair of parallel electrodes, and function to store electricity utilizing an electrical polarization caused in the dielectric by application of a voltage between the parallel electrodes. The characteristics of the capacitors largely vary depending on the kinds of dielectrics used. At present, ceramic capacitors using dielectric ceramics, typically barium titanate, semiconductor ceramic capacitors utilizing semiconductive properties produced by addition of lanthanum oxide to barium titanate, film capacitors using plastic films such as polystyrene or polyethylene terephthalate as dielectrics, and aluminum electrolytic capacitors using as the dielectric layer a porous layer produced by anodic oxidation of the surface of highly pure aluminum foils have been used properly in accordance with purpose in consideration of temperature coefficient of capacitance, insulation resistance, dielectric loss and high-frequency characteristics of capacitors.
In recent years, demands for high intergration and high performance of electronic devices and equipments are increasing more and more. Accordingly, miniaturization, lightening, multifunctionalization and high reliability have been demanded also with respect to capacitors. However, various problems are encountered in achieving these demands. For example, a problem of heat release encountered in high integration must be solved. This problem may be dealt with by circuit design and packaging technique, e.g. low power operation and heat releasable structure, but basically the solution by realization of capacitors resistant to high temperature operation is required. Also, high integration always includes high speeding of circuit operation, and it is becoming very important to meet an increase in frequency of dealt signals. For example, in computers which are a representative example of electronic devices and equipments, operating frequency increases more and more, and even in usual personal computers an operating frequency over 20 MH.sub.z tends to become common. Also, in the field of satellite communication or the like, operation at gigahertz (GH.sub.z) band is demanded. In such fields, it is requested that capacitors themselves do not generate waveform distortion and noise also in a high frequency region. Also, since the number of terminals increases inevitably with increase in the degree of integration, it is necessary to decrease pitches between the terminals and, therefore, new connecting means and arraying have also been demanded.
In addition to high integration, countermeasures for miniaturization and lightening have also been demanded, and it may safely be said that thin film capacitors are indispensable to production of capacitors which have a large capacitance even if they are of small area and thin types.
Paying attention to improvement in performances of capacitors required for high integration, miniaturization and so on, capacitors widely used at present have the following problems.
(1) Ceramic capacitors have the defect that the capacitance largely varies depending on temperature. Also, for obtaining a large capacitance, it is necessary to built-up thin pieces of ceramics and, therefore, ceramic capacitors have a limit in miniaturization. Further, the arraying makes process step complicated, since the arraying must be made by preparing individual built-up type capacitors and adhering them with alignment onto a common substrate such as ceramic substrate or epoxy substrate.
(2) Semiconductor capacitors have the defects that the capacitance largely varies depending on voltage and the voltage resistance is poor, in addition to the same problems as those encountered by ceramic capacitors in miniaturization and arraying.
(3) Film capacitors have an inductance and are not suitable for use at high frequency. They also have the problems that the capacitance is relatively small and the temperature of use is restricted by the kind of plastics used.
(4) Aluminum electrolytic capacitors have the problems that the applicable frequency is as low as 1 MH.sub.z or less and the temperature coefficient of capacitance is large. Also, judging from the process steps for the preparation, the arraying would be difficult.
It is a primary object of the present invention to provide a small size capacitor which can meet high integration and miniaturization of electronic devices and equipments and which has excellent high frequency characteristics as usable in a high frequency region and has a good heat resistance.
A further object of the present invention is to provide a thin film capacitor which is stable in capacitance against changes of temperature and frequency and which can meet the production of a capacitor array of high quality.
These and other objects of the present invention will become apparent from the description hereinafter.